I just beat Sirius.
"When the five stars shine together, you two and Earth will see your final days!"
Turns out dummied out cards exist in Star Force games, too. Like a red Cannon card (1), nameless cards (1; I think two of them are Lunar Knight characters of the Boktai-world?).
Then there's blue-name cards (2 and 3). From 3, I found some like "ImpctCanon", "ApolloFlmX", GenAurigaX" and "DragonSkyGX"...
(I used an all-cards cheat just to see if there were dummied-out cards, but didn't save.)
I already know about the unused Lunar Knight cards, turns out they're related to the cut Lunar Knights sidequest. Also in Star Force 3, there are cards known as Illegal Cards. They're powerful cards which can be obtained if your Noise Level is high enough and are named in blue to differentiate them. They're very useful for the post-game bosses.
Hopefully, you'll have an easier time getting Sirius v2's card than I did. Let's just say that having to obtain that card from Sirius v3 is utterly
painful. As is obtaining both Apollo Flame's v2 and v3 cards. Also Crimson Dragon SP is much harder than it's regular self.